α-ionone is a compound found from various essential oils. It has a floral that is stronger than that of β- or γ-ionone. Racemic α-ionone has a violet-like flavor (aroma), and it is used as a perfume. On the other hand, optically active α-ionone, (R)-α-ionone and (S)-α-ionone have been found in the nature. The former (R) form has a unique and strong floral flavor (aroma), such as a violet-like, fruit-like, raspberry-like flavor. The latter (S) form has a unique and strong flavor (aroma), such as a fresh juicy greenish flavor, including a wood-like, cedarwood-like, raspberry-like, and β-ionone-like flavor (aroma).
As a method for producing such optically active α-ionone, Patent Document 1 describes a method for producing optically active α-ionone in 5 steps using 2,4,4-trimethyl-2-cyclohexen-1-ol as a raw material. However, the method described in Patent Document 1 contains a step of allowing optically active 2,6,6-trimethyl-2-cyclohexen-1-ylacetaldehyde obtained as an intermediate to react with potassium cyanide. Thus, this method has been problematic in that it has poor workability and in that it is unfit for an industrial production.
In addition, Non-Patent Document 1 describes a method for producing (R)-(+)-α-ionone and (S)-(−)-α-ionone, that is optically resolving α-cyclogeranic acid to give (R)-(+)-α-cyclogeranic acid and (S)-(−)-α-cyclogeranic acid, and then using these compounds as raw materials. The method, however, described in Non-Patent Document 1 requires 4 steps. Thus, this method is poor in terms of yield and efficiency, and it is unfit for an industrial production. Moreover, Non-Patent Document 1 describes a method for purifying (R)-α-ionone in total 5 steps, that is epoxidation of the double bond of the cyclohexene ring of (R)-α-ionone. This method, however, includes 5 steps, and thus it has been problematic not only in that it is expensive, but also in that it is unfit for an industrial production.
Patent Document 1: Japanese Patent Laid-Open No. 10-84989    Non-Patent Document 1: Agric. Biol. Chem., vol. 51, No. 5, 1271-1275 (1987).